1. Field of the Invention
The present invention relates to short range wireless communication systems. More particularly, the present invention relates to short range inductively coupled communication systems having an expanded channel capability.
2. Description of Prior Art
Short range wireless communication systems are of significant importance for a variety of applications. Although many applications are quite common, such as cordless telephones, "walkie talkies" and cordless microphones, a large number of other potential applications also exist. Such potential applications, which could include virtually any system employing audio cord coupling, have generally not been exploited due to limitations in existing short range communication systems.
There are four basic wireless communication systems in use today: acoustic, radio frequency, infrared and inductive. Each of these has performance characteristics, and attendant advantages and limitations, well known to those skilled in the art. For example, acoustic communication is short range, can cover large groups of people and is subject to interference. Radio frequency (or RF) communication in turn is long range and can use many forms of modulation including amplitude modulation (AM) and frequency modulation (FM) for transmitting signals on an RF carrier. Infrared communication is generally limited to short range, line of sight communication and is immune to forms of interference other than light. The cost vs. range vs. fidelity of each method is well known to those skilled in the art.
The last noted approach to short range wireless communication employs inductively coupled, or magnetically coupled, wireless transmission. In particular, this approach has been employed in hearing assistive devices for communication between a belt worn radio and a small hearing aid of the type worn behind the ear (BTE) or in the ear (ITE). Cordless coupling between the belt worn device and the ITE or BTE hearing aid is important both for aesthetic reasons, in view of the fact that hearing aid wearers usually desire to avoid calling attention to their hearing disability, as well as for practical reasons relating to awkwardness relative to a cord directly attached to a BTE or ITE hearing aid.
In such a hearing assistive application, the belt worn receiver or microphone is coupled via a cord or cable to a loop which is worn under the clothing of the hearing aid user and encircles the user's neck. The ITE or BTE hearing aid in turn has an inductive pick up loop therein, for example, the inductive loop employed for picking up telephone audio (so called teleloop, T-loop or T-coil), to pick up the audio signal from the neck worn transmission loop. The audio signals from the belt worn audio source directly amplitude modulate the current through the neck worn loop to produce an amplitude modulated signal in the audio frequency band. This is commonly referred to as a base band amplitude modulation system, since no carrier frequency is employed (as in the case of RF transmission). The use of an inductive loop allows connecting cords to be omitted from the ITE or BTE hearing aid thus providing the advantages of aesthetics, i.e., removing visible cords from the hearing impaired user, as well as convenience due to the lack of cords which may become tangled or interfere with the actions of the user.
One advantage of an inductively coupled system relative to an RF transmission system is that since that such an inductively coupled system operates in the audio frequency range, FCC regulation is not applicable and problems related to FCC licensing are avoided. Another advantage of inductive coupling for short-range communication systems of this type is that low power consumption is associated with the system. Due to the desired small size of the hearing aid, low power consumption is a key requirement since any batteries employed must be of small size and hence limited in power supply capabilities. Also, the transmitter portion of the system, in the noted example a belt worn device, may be battery operated and hence sensitive to power consumption.
A significant disadvantage of an inductively coupled base band amplitude modulated system is the tendency to experience signal variations due to changes in the relative orientation of the receiving coil and the transmission loop as well as sensitivity to distance between the receiving coil and transmission loop. This not only causes fading in and out of the signal as the user moves about, but also creates serious signal to noise problems where lower magnitude audio signals are being transmitted.
In addition, the electrical inductance of either the transmitting or receiving coil is inversely related to the bandwidth of the system. Increasing the electrical inductance reduces the bandwidth, providing a "Catch-22" where wide bandwidth low power transmission is desired.
Another problem of base band inductively coupled systems is the high background noise typically present due to the magnetic fields of common electrical devices. A base band magnetic element (pick up coil) typically has its resonance in the audio range, and a typical system has a sensitivity around 2 milliGauss (mG). Electrical appliances, motors, fluorescent lights produce a magnetic field in the 60-120 Hz range, which overlaps the audio range and can be detected in base band operation. The following are typical magnetic field strength of a variety of electrical devices, measured at a distance of 3 feet.
______________________________________ Fluorescent Lights 3 mG Electric Toaster 3 mG Electric Motor 2 mG House Wiring 2 mG Electric Blanket 100 mG ______________________________________
When compared to an inductive coupled base band system sensitivity of about 2 mG, it is clear these items have enough of a magnetic electric field to cause significant interference in base band systems.
There are thus several serious limitations associated with base band systems: signal fading with varying distance and orientation of the receiver with respect to the loop; signal to noise ratio deterioration with distance; restrictions on the signal bandwidth by parameters of the circuit elements; and presence of high background noise due to common electrical devices. Therefore, the quality of audio communication provided by base band systems has been less than desired.
As a practical application of a short range communication system, the replacement of corded headsets with cordless headsets for communications between the headset and a base station has known practical advantages.
Traditional corded headsets for use in two-way communication provide the user with noise reduction from the surrounding environment as well as privacy. Corded headsets suffer, however, from numerous disadvantages including excessive weight and size which leads to wearing discomfort after several hours of use. Additionally, cords present another problem for headset users. Cords tend to tangle, break, and limit the area of operation of the headset. Also, corded headsets often require electrical isolation from the main panel or console as a preventative measure against electrical shock.
Advantages afforded by cordless headsets are many and include the capability of providing the wearer with flexibility as to physical movement, in addition to acceptable levels of signal to noise performance. The ability to conduct two-way communications while still having the freedom of using both hands is a primary benefit of the cordless headset. Air traffic controller, stock brokers, secretaries, busy executives, telemarketers, operators, receptionists, and others often use headsets in everyday applications. A cordless headset would enable such persons to enjoy much more flexibility in terms of movement. Existing cordless headsets, however, often can cause the wearer discomfort due to their large size and weight. Current power requirements for cordless headsets contributes to the uncomfortably large size of the currently available headsets. While the cost of the available cordless headsets tends to vary, the circuitry associated with the needed transmitter and receiver components, and power requirements, have kept cordless headsets uniformly more expensive than corded headsets. In addition, privacy considerations often limit the use of cordless headsets since it is possible for third parties with a receiver tuned to the proper frequency to listen in on communications between a base station and the headset.
The typical cordless headset provides for two-way communications between a headset and a base unit. The two-way communications typically use only one mode of transmission: radio frequency, infrared or magnetic coupling.
RF headsets are the most commonly used for two way communications. In an RF headset, both the receiver and transmitter are RF. Two accomplish full duplex communications, a separate transmit and receive channel are required. Because of the difficulty in controlling RF radiation patterns, additional channels are often required. However, due to the limited number of available channels in the radio frequency spectrum, users are required to share the available channels. In addition to causing interference, this places restrictions on the number of users in a given area.
Designers of RF headsets often try to make greater use out of the limited number of RF channels. Spread spectrum and frequency synthesis are two popular methods that make greater use of the available channels. Such techniques have shown promise in making optimum use out of the available number of RF channels, however, both techniques require complex circuitry which consumes space and power, thus adding to the size and weight problems discussed above. In addition, such circuitry adds to the cost of the system.
Further, in RF headsets, Frequency Modulation, or "FM," is the preferred method for modulating the RF carrier. As those skilled in the art w! ill appreciate, FM modulation delivers a higher quality signal than Amplitude Modulation or "AM." Of course, the detection scheme for FM modulation is more complex than that for AM, and thus requires more circuitry. The FM modulated carrier typically operates at a duty cycle greater than 50% and therefore requires more power for transmission than a signal that would be modulated at a lesser duty cycle. Further, unless expensive coding techniques are used, the transmission of RF signals between the base station and the headset enables third parties to intercept the transmitted signals, thus creating additional concerns regarding the security of the communications.
Infrared headsets are not susceptible to interference problems associated with limited channel availability as are RF headsets, but are restricted to line of sight communication. While line of sight communications provide for a relatively secure communication link, thus relieving some privacy concerns, infrared communications can require a considerable power supply. For example, the amount of power required by an infrared headset is proportional to the ambient light conditions in the user area and the range or distance from the base station. Thus, as the ambient light level increases, the power that is needed to overcome the ambient light noise floor to complete the transmission from the headset to the base station also increases. Similarly, as the distance from the headset to the base station increases, the power required to transmit a signal from the headset to the base station also increases.
While allowing significantly more freedom of movement than is possible with corded headsets, infrared systems suffer some mobility limitations due to the line of sight requirement associated with using infrared beams. If the infrared beam between the base station and the headset is blocked, the user will be unable to maintain a communication link. In situations where communication integrity is essential, for example, air traffic control, the possibility of a base station/headset communication link interruption may prevent the use of infrared cordless headsets.
Magnetically coupled headsets offer limited performance within a given area but are highly susceptible to the magnetic interference from fluorescent lights, motors, computer/CRT monitors, video monitors and any large power generator producing a magnetic field. Magnetically coupled headsets are also susceptible to large changes in signal strength if the distance or orientation with respect to the base station is varied. In addition to susceptibility to signal level variations, a large amount of power is required to transmit a magnetically coupled signal over a given distance. Such power requirements can limit the operating life of the headset battery or require larger and heavier batteries to maintain a reasonable operating period between battery recharge or replacement.
A common denominator among the above-discussed three primary methods of signal transmission between two-way cordless headsets and a base station is concern over the power requirements for the headset. As one skilled in the art will appreciate, battery power for a cordless headset will generally be the determining factor as to the size, weight, operating life and range or distance over which the headset may be used. Further, the size and weight of the headset will directly impact the ergonomics of such a device, and thus the comfort level of the user wearing the headset.
Accordingly, it will be appreciated that a need presently exists for a short range wireless communication system which consumes relatively little power and which can be implemented without complex circuitry. It will be further appreciated that a need presently exists for a short range wireless communication system which provides good audio quality reception and which is not subject to the above-noted problems. It will be further appreciated that a need presently exists for a short range wireless communication system which is not subject to federal regulations involving long range communication systems.
Further, as the above discussion illustrates, a need currently exists for a very low power consumption cordless headset that includes expanded channel capability in communicating with a base station. Such a headset must be comfortable enough for extended wear, yet also provide longer battery life, and provide equivalent or improved signal to noise ratios and interference resistance than that currently available with the above-discussed transmission modes.